Quantum Spin Liquid Physics in Atomically Thin -RuCl3 Enables Exploration of 50% Excitations

The search for materials exhibiting quantum spin liquid behaviour represents a significant frontier in modern condensed matter physics, offering potential pathways towards fault-tolerant quantum computation. Claudia Ojeda-Aristizabal from California State University Long Beach, Xiaohu Zheng from the Beijing Academy of Quantum Information Sciences, and Changsong Xu, alongside Zohar Nussinov from Washington University in St. Louis, Yukitoshi Motome from The University of Tokyo, and Arnab Banerjee from Purdue University, investigate how insights from the material alpha-ruthenium trichloride (-RuCl3) can advance the pursuit of these exotic quantum states in atomically thin materials. Their work focuses on understanding how Kitaev magnetic interactions, crucial for forming spin liquids, manifest in layered and exfoliated -RuCl3, and how this knowledge can be applied to engineer superior two-dimensional materials. A key finding reveals that controlling charge transfer between -RuCl3 and neighbouring materials can substantially enhance these Kitaev interactions, potentially bringing researchers closer to realising elusive spin liquid phases and unlocking new possibilities for quantum technologies.

Realising these states proves challenging due to the natural tendency of magnetic interactions to create conventional magnetic order. This work investigates the layered material α-RuCl3 as a platform for exploring QSL physics and developing strategies to suppress magnetic order. Researchers examine how reducing dimensionality, through exfoliation into atomically thin flakes, impacts magnetic properties and the potential for realising a QSL state.

The team demonstrates that thinning α-RuCl3 significantly weakens long-range magnetic order, bringing the material closer to a QSL phase. They achieve this by fabricating and characterising exfoliated flakes with thicknesses ranging from several nanometres to hundreds of nanometres. Detailed measurements of magnetic susceptibility and Raman scattering reveal a suppression of three-dimensional magnetic order and the emergence of novel low-energy excitations. These excitations, indicative of fractionalised spin degrees of freedom, represent key signatures of a QSL state. Furthermore, the study identifies the crucial role of interlayer coupling in stabilising magnetic order in bulk α-RuCl3.

By reducing the number of layers, the interlayer exchange interactions weaken, allowing quantum fluctuations to dominate and suppress magnetic ordering. The team finds that even a few layers are sufficient to significantly reduce the ordering temperature and enhance the QSL character of the material. This research provides valuable insights into the design of atomically thin materials for hosting QSL phases and paves the way for exploring exotic quantum phenomena in reduced dimensions. The findings establish a clear pathway for manipulating magnetic interactions and achieving QSL behaviour in van der Waals materials.

Kitaev Interactions and Magnetic Ordering in α-RuCl3

Ongoing research focuses on understanding whether α-RuCl3 exhibits the properties of a Kitaev spin liquid, a highly unusual state of matter predicted by theory. This involves identifying and characterising the magnetic interactions within the material, determining the strength and nature of the exchange interactions, and searching for signatures of the Kitaev spin liquid, including fractionalised excitations, gapless spin liquid behaviour, and a thermal Hall effect. Researchers also investigate the role of structural distortions and imperfections, and the effects of magnetic fields on the material’s properties. The research relies heavily on advanced experimental techniques, including neutron scattering to probe magnetic structure and excitations, resonant inelastic X-ray scattering to study magnetic excitations and electronic structure, terahertz spectroscopy to investigate low-energy magnetic excitations, and thermal transport measurements to search for Majorana fermions.

Magnetic susceptibility and specific heat measurements characterise the magnetic response and low-temperature behaviour, while X-ray diffraction determines the crystal structure and identifies structural distortions. Computational methods, including first-principles calculations and machine learning, complement the experimental studies. Specific research areas include determining the precise nature of magnetic interactions and anisotropy in α-RuCl3, understanding the impact of structural distortions on magnetic properties, and investigating the effects of magnetic fields. Some experiments have reported evidence for fractionalised excitations and a thermal Hall effect, although these results require further confirmation.

Researchers are also investigating whether α-RuCl3 exhibits gapless spin liquid behaviour, characterised by the absence of magnetic ordering at low temperatures. Machine learning techniques are being used to analyse experimental data and extract interaction parameters, and theoretical studies are used to understand experimental results and predict new phenomena. Several challenges and debates remain in the field, including ensuring sample quality, interpreting experimental data, distinguishing between different spin liquid phases, and understanding the role of disorder. Ongoing research continues to shed light on the complex magnetic properties of α-RuCl3 and its potential as a Kitaev spin liquid candidate.

Enhanced Kitaev Interactions in Layered Materials

Recent research demonstrates significant progress in identifying and characterising materials that exhibit Kitaev magnetism, a phenomenon with potential applications in topological quantum computation. Investigations into layered materials, particularly ruthenium-based compounds, have revealed that charge transfer at interfaces can substantially enhance Kitaev interactions, improving the prospects for realising these exotic magnetic phases. Specifically, studies of ruthenium trichloride demonstrate a significant increase in Kitaev coupling, up to 50%, when combined with other two-dimensional materials. Beyond ruthenium compounds, researchers have expanded the search to include cobalt and nickel-based systems, as well as rare-earth materials.

Analyses of sodium cobalt oxide reveal a substantial Kitaev coefficient, alongside measurable Heisenberg and off-diagonal terms, contributing to a more complete understanding of magnetic interactions within the material. Furthermore, computational surveys of rare-earth compounds, such as those containing praseodymium, predict strong antiferromagnetic Kitaev couplings, offering a complementary route to realising Kitaev physics beyond traditional d-electron materials. While some cobalt-based systems remain controversial, with questions surrounding the sign and even existence of Kitaev interactions, these investigations collectively broaden the scope of potential materials for hosting Kitaev magnetism. Further research is needed to fully understand the complex interplay of magnetic interactions in these materials, particularly in the cobalt-based systems where results are currently debated. Future work will likely focus on refining theoretical models and conducting additional experiments to confirm the presence and strength of Kitaev interactions, ultimately paving the way for the development of materials that can reliably exhibit and harness the unique properties of Kitaev spin liquids.